Really important for the beginners starting with zimbra, which define what is meant by a small, medium, large and very large companies, in terms of number of users, domains and subdomains, consumption, concurrent users, etc. In my case it was to implement a multiserver installation for 400 users thinking it was the most suitable, then vi: http://etomicmail.com/services/zimbr..._Benchmark.pdf and I realized that a single server will super-fine.